This invention relates to an image forming apparatus.
Conventionally, an electrophotography is used in an image forming apparatus such as a copier, a printer, a facsimile or a combined apparatus that combines the functions of copying, printing and faxing. Such an image forming apparatus performs processes of charging, exposure, developing, transferring, fusing and cleaning. As disclosed in Japanese Laid-Open patent publication No. 10-138549, the above described processes are respectively performed by a charging unit, an exposing unit, a developing unit, a transfer unit, a fixing unit and a cleaning unit.
The charging unit includes a conductive charging roller to which a direct voltage is applied. The charging roller contacts a photosensitive body to uniformly charge the photosensitive body. The exposing unit exposes the photosensitive body to the light so that a latent image is formed on the photosensitive body.
The developing unit includes a developing roller made of a resilient conductive material to which a direct voltage is applied, and a toner supplying roller that supplies the toner stored in a toner container to the developing roller. The developing roller contacts the photosensitive body so that the toner adheres to the latent image on the photosensitive body.
The transfer unit includes a transfer roller made of a semiconductive sponge to which a direct voltage is applied. The transfer roller contacts the bottom side of the recording sheet opposite to the photosensitive body so that the toner image is transferred from the photosensitive body to the recording sheet.
The cleaning unit includes a cleaning blade that contacts the surface of the photosensitive body to recover the residual toner that remains on the photosensitive body after the toner image is transferred to the recording sheet.
The conventional image forming apparatus has a resolution of 300 dpi. However, in order to meet the recent demand for improving image quality, the image forming apparatus is required to have a resolution of 600–1200 dpi or more, and therefore it is necessary to make improvements in the electrophotographic processes. Particularly, it is studied that the resolution can be improved by reducing the size of the toner particle or by reducing the thickness of a charge transport layer of the photosensitive body. However, it is difficult to reduce the size of the toner particles because of the difficulty in manufacturing. Moreover, it is difficult to reduce the thickness of the toner layer because the lifetime of the charge transport layer may be shortened with decreasing thickness.
Moreover, it is known that the scattering of the toner particles may occur when the toner particles are transferred from the photosensitive body to the recording sheet. The scattering of the toner particles is considered to be caused by electrostatic force acting on the toner particles at the transferring position. As the resolution increases, the gap between the smallest dots formed on the recording sheet decreases, and therefore the scattering of the toner particles may easily effect the image quality. For example, under the assumption that the allowable range of the scattering of the toner particle is within 150% of the size of the smallest dot, the scattering of the toner particles must be restricted within an area with the diameter of 126 μm when the resolution is 300 dpi. However, if the resolution is 600 dpi, the scattering of the toner particles must be restricted within an area with the diameter of 63 μm. Further, if the resolution is 1200 dpi, the scattering of the toner must be restricted within an area with the diameter of 32 μm.